Matt:
Rest assured, there is no way that set of authors have played all 1000 of the courses they will list. I'd bet a hundred euro that they have not even played half of them.
But, if there really was a consensus list of the top 1000 courses in the world, I would be willing to wager I only know one person who has PLAYED more than half of them (I might have seen more than half, but I certainly haven't played that many). Can you think of anybody who would have? I guess if fully half of them are in America, then there are a few people who would have played 400 out of those 500, and maybe 100 others abroad.
I agree with Mark B. that the magazines are all compromised in the lists they put out, but at least they have some incentive to try and get out a better list than their competitors. Rolex has no real incentive to get their list right, and it's highly unlikely they would spend the time or money to double-check themselves. Most likely they will just list every golf course that's ever made a top 100 list, plus all of the GOLF DIGEST international listings, and then round out the list with their own favorites and those in which their friends have a financial interest.